In recent years there has been active research in the area of nanosecond-type pulse generation. Such research has produced GaAs substrate high power storage devices that utilize photoconductive solid state switching to generate nanosecond-type pulses. One such switch, disclosed in U.S. Pat. No. 5,028,971, issued to Anderson H. Kim et al on Jul. 2, 1991, entitled, "High Power Photoconductor Bulk GaAs Switch" is incorporated herein by reference.
The "Bulk GaAs Switch" is basically an electrical energy storage device comprised of two mutually opposite gridded electrodes positioned on opposite surfaces of a GaAs semiconductor substrate such that a power supply means can provide an electric field in a predetermined direction across the patterned electrodes. The device is photoconductively activated to discharge its stored energy when it receives light radiation at a predetermined wavelength. When light energy penetrates the substrate region it generates electron/hole pairs which cause the electrical resistance of the semiconductor material to decrease. As a result, the stored electrical energy will instantaneously discharge through a load.
It is widely recognized that when such devices discharge they radiate pulses in a direction perpendicular to the substrate surface. The bandwidth of such pulses is determined and/or limited by the speed with which the device discharges and recovers. As the pulsewidth narrows, the bandwidth increases. Consequently, those skilled in the art widely recognize the benefits of a device that generates a narrow pulsewidth.
The critical elements in generating such narrow pulsewidths are the construction of the energy storage device (shape, size, etc.) and the effectiveness of the photoconductive switching. Heretofore, two general techniques have been used to generate such narrow pulsewidth (ultra-wideband) radiation.
The first technique utilizes the recombination property of the semiconductor material from which the switch itself is fabricated. This technique (using photoconductive GaAs switches), however, typically generates a signal with a long pulsewidth due to a relatively long recovery time. The long recovery time is attributed to the inherent properties of gallium arsenide, including: (1) the substantially long recombination time and (2) the switch lock-on phenomena. As such, this technique is not desirable for generating ultra-wideband pulses.
The second technique utilizes an energy storage element which is comprised of either a short section of transmission line or a capacitor. The energy storage element is photoconductively triggered to instantaneously discharge all or substantially most of its stored energy to an impedance load. As with the aforementioned technique, the extended recovery time inherent in photoconductive switches prevents this device from producing extended wideband radiation.
A major breakthrough in this pulsewidth problem, however, was presented in the inventors copending patent application entitled "Ultra-Wideband High Power Photon Triggered Frequency Independent Radiator," Ser. No. 07/946,718, filed by Kim et al, Sep. 18, 1992 and incorporated herein by reference. This frequency radiator combines an energy storage function and an antenna radiating function into one structure to create an ultra wideband frequency radiator capable of generating pulses with a range of frequency components from hundreds of megahertz to several gigahertz. Basically, this radiator utilizes two identical quasi-radial transmission line structures to store electric energy while it implements photoconductive switching to trigger the instantaneous discharge of the stored energy to generate the desired ultra wideband RF radiation.
Such an energy storage device comprises a dielectric storage medium, two quasi-radially shaped, metalized electrodes mounted opposite one another on the top surface of the dielectric storage medium and a metalized electrode mounted on the bottom surface of the dielectric medium. A photoconductive switch, centrally located on the dielectric between the two quasi-radially shaped electrodes, connects the two quasi-radially shaped electrodes to the bottom electrodes through a load impedance. When the switch is activated by light radiation, the stored energy discharges through the load impedance generating a sub-nanosecond-type pulse.
Although such a device provides for fast rise-time pulses, the radiation bandwidth is limited by the trigger speed of the photoconductive switch and the recovery time of the GaAs substrate. One method of breaking through these physical limits and thus increasing the radiation bandwidth is by sharpening the discharge pulse. A sharpened pulse is a fast rise-time as well as fast fall-time pulse. Consequently, those skilled in the art recognize and desire the bandwidth benefit resulting from an Rf radiator that incorporates pulse sharpening techniques.